The present invention relates to an intrusion warning alarm system for sensing and indicating the presence of an intruder within a given area. More particularly, the present invention relates to an improved intrusion warning system of the type wherein the presence of an intruder within a given area is detected by determining changes caused in an electric field caused by the movement of the intruder.
The basic type of intrusion warning or detection system which responds to changes in an electrical field caused by an intruder, and to which the present invention relates, is disclosed in U.S. Pat. No. 3,237,105 issued Feb. 22nd, 1966 to Henry P. Kalmus, the subject matter which is incorporated herein by reference. According to the teachings of this patent, a transmitting electrode and a receiving electrode are positioned on opposite sides of an area to be protected, and a quasi-stationary electric field is produced within the area to be protected by supplying a signal at a frequency in the range of, for example, 5-100 KHz to the transmitting electrode. The receiving electrode is connected by an amplifier to an AM detector which detects the modulation of the received field signal caused by the movement of an intruder through the electric field between the transmitting and receiving electrodes, and the detected signals fed to a bandpass amplifier having a passband in the order of 2-20 Hz, thereby passing the low frequency component due to movement of an intruder in an area between the electrodes. This filter output signal is then fed to an indicating or alarm device to indicate the presence of an intruder.
While the system disclosed in this patent operates satisfactorily in principle, it is susceptible to a number of problems when attempting to utilize same in practical applications or to extend the range of the system so that it can be used to cover relatively large areas, thus requiring that the sensitivity and hence the gain, be increased. One primary problem in practical applications in such distance is that of false alarms caused, for example, by stray electric fields within the passband of the system, by transients resulting from relative movement of the electrodes and/or, particularly when such a system is used outdoors, by movement of animate or inanimate objects, for example, animals or large bushes, in an area adjacent to generate an electric field.
U.S. Pat. No. 4,064,499 issued Dec. 20th, 1977 to T. D. Geiszler et al, subject matter which is likewise incorporated herein by reference, describes an improved intrusion warning system of the basic type identified above wherein the susceptibility to false alarms is materially reduced, and wherein the versatility of the system with regard to its application is materially increased. According to the teachings of this patent, these advantages are achieved by various features or combination of features, for example, a more desirable frequency range for the generated field, improved filtering, and a need for the detected signal exceeding a given threshold value for a given period of time before an indication or alarm is initiated. Although this system operates very satisfactorily for many applications, it still is susceptible to false alarms from certain types of external signals and/or moving objects. Additionally, particularly when the intrusion warning system is being utilized for perimeter type protection, it is desirable to know the direction of the intrusion, i.e. whether or not the intruder is entering or leaving the area being protected, which information is not provided by the system according to the patent.